Muscle stimulator



FIG. 1 is a side elevational view of a muscle stimulator, showing my new design, the opposite side being a mirror image thereof;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a bottom view thereof. 

The ornamental design for a muscle stimulator, substantially as shown and described. 